Method of developing electrostatic images



Filed Nov. 1, 1955 N'N N INVENTOR. JOSEPH C. WILSON JTZ'OJ'UVEY METHOD OF DEVELOPING ELECTROSTATIC IMAGES Joseph C. Wilson, Rochester, N. Y., assignor to Haloid Xerox Inc., a corporation of New York Application November 1, 1955, Serial No. 544,174

2 Claims. (Cl. 11717.5)

My invention relates to xerography and more particularly to an improved method of developing an electrostatic image. The principal object of my invention is to form a better image in the finished print, particularly in the solid-area portions thereof.

In xerography it is usual to form an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation. Other means of forming electrostatic latent images are set forth in U. S. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electroscopic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternately, the electrostatic charge pattern may be transferred to an insulating film and the electrostatic particles deposited thereon to form the visible image. In any case, this visible image in turn may be transferred to a second surface to form a xerographic print or may be fixed directly to the photoconductive surface.

The usual process of applying the developer to the latent electrostatic image is set forth in U. S. 2,618,552 and involves the use of a finely-divided colored material, called a toner, deposited on a slightly more coarselydivided material called a carrier. This two-component developer is cascaded across the electrostatic image areas. The toner and carrier, being rubbed against each other while cascading, impart an electrostatic charge to each other. This process is known as triboelectric charging. To produce a positive of the electrostatic image a toner and carrier are selected such that the toner will be charged to a polarity opposite to that of the electrostatic image, the carrier being charged to the same polarity as the electrostatic image. When a carrier particle bearing on its surface oppositely charged particles of toner crosses an area on the image surface having an electrostatic charge, the charge on the surface exerts a greater attraction for the toner than the carrier and retains the toner in the charged area and separates it from the carrier particles. The carrier particles being oppositely charged and having greater momentum are not retained by the charged areas of the plate. When a toned carrier particle passes over a non-image, i. e. non-charged, area of the plate, the electric attraction of the carrier particle for the toner particles is suflicient to retain the toner on the carrier preventing deposition in such areas as the carrier particles momentum carries both toner and carrier past. By this mechanism the image is developed, i. e. made visible.

I have found that I can obtain superior reproductions using a single magnetic material to serve the dual function of carrier and toner. Thus, superior solid area coverage is obtained; only one material need be handled, stored and processed; and finally the developer has the unique property that either positive or negative reproductions of United States Patent fitice 2,846,333 Fatented Aug. 5, 1958 2 an original may be made with equal ease by simply applying the proper electric potential to the magnet.

In carrying out my invention, the developer, in addition to being magnetic and having the proper distribution of particle size, should be of low electrical conductivity so as to avoid objectionable discharge of the latent image by the conductivity of the developer. I have found that certain ferromagnetic materials known as ferrites have the desired properties for use in the developer particles of my invention. These materials have a spec fic resistance higher than 1,000 ohms-cm. They also have high initial permeability. Examples of such ferrites are disclosed in U. S. patents to Jacob Louis Snoek, Nos. 2,452,529; 2,452,530; and 2,452,531; all dated October 26, 1948. They have the composition, MeFe O where Me is a bivalent material such as Mn, Co, Ni, Cu, Mg, Zn or Cd. They have the same crystal structure as Fe O These ferrites are prepared by a sintering process such as that used in the ceramic .industry. The component metal oxides are ground, sintered and reduced to proper size. Examples of suitable magnetic ferrites which have been found satisfactory are the materials known as H1 and C-159, Ferramic powder, manufactured by General Ceramic Corporation, Keasby, New Jersey; Plast Iron powder, AIBAIO manufactured by Plastic Metal Division of the National Radiator Company, Johnstown, Pennsylvania; and the material known as MP25 manufactured by Magnetic Powders, Inc., Johnstown, Pennsyl- Vania; I have obtained improved results with these and other ferrites. The initial permeability of the materials used by me, so far as known to me, ranged from to 1200, the maximum permeability from 183 to 4800. Their specific resistances ranged from 1x10 to 2x10 Good results have been obtained with developers ranging in size from to 325 mesh. This is not critical and even larger particle sizes may be used. It is important, however, that an appreciable portion of the particles have a particle size of no more than about 20 microns together with another appreciable portion of the particles being of such a weight that they are movable by gravity away from the finer particles adhering to the charged areas on the plate. In general, the ratio of fines to the rest of the particles is not at all critical and may vary over a wide range. Too small a portion of fines results in excessive development time; while too high a portion is apt to cause undesirable deposition in background areas. In general, the percentage of fines to the total powder should be from about 5 to about 35% by weight with an average of about 20% by weight. This developer may be applied to the surface containing the electrostatic image by a variety of means.

Referring to the drawings, Fig. l is an elevation of a hand device for applying the devolper to the image. Fig. 2 is an end view thereof. Fig. 3 is a top plan view of apparatus for applying the developer to the image, and Fig. 4 is a section taken on line 44 of Fig. 3.

I have obtained good results with the magnetic brush construction as shown in Fig. l and 2. This brush comprises a plurality of magnets 1 which are arranged side by side with corresponding poles adjacent each other. For example, in Fig. 1 all the north poles of magnet 1 are at the upper end of the brush and the south poles at the lower end. The magnets 1 are secured in any suitable way to the soft iron pole pieces 2 and 3 adjacent the respective poles. The pole piece 3 is preferably made .with its lower portion of pointed or V-shaped cross section as shown in Fig. 2.

In using the above described magnetic applicator, the magnetic developer particles are placed in a suitable trough, for example, a metallic trough of V-shaped cross section (not shown). The lower pointed end of the brush is then dipped into the developer. Because of the magnetic nature of the developer particles, the brush picks up the developer so that the developer particles adhere thereto in the form of the bristles of a brush (whence the name). The brush is then pressed upon the surface containing the latent electrostatic image so that the said surface is just touched by the developing material.. After this, the magnet is moved back and forth on the said surface until the image is developed to completion. The fines of the developer adhere to the charged image and the larger particles are removed with the applicator.

In Figs. 3 and 4 is shown one form of apparatus for magnetically applying the developer to the surface bearing the latent electrostatic image.

A shaft 4 mounted in bearings 5 and 6 has a plurality of spaced discs 7 secured thereto at an angle to the axis of the shaft. Magnets 8 and 9 (which may be made of Alnico) have their south poles connected by pole piece 10. The north poles of the magnets are connected magnetically to the shaft 4 through soft iron members 11, 12 and 11', 12' respectively. The edges of the discs should be in close proximity to the pole piece 10. Under the discs 7 is a trough 14 arranged to support the developing powder 17 so that the discs dip therein. The top and forward portions of the pole piece may be covered by a sheet 15 of brass so as to provide a suitable bearing surface against which the plate 16 carrying the electrostatic image may be readily moved in a vertical direction. A layer of dielectric material 18, such as polyethylene terephthalate, is placed between the brass sheet 16 and pole piecelt). Any suitable means (not shown) may be used to rotate the shaft 4 and to move the plate 16.

In operation, a magnetic circuit passes from the north poles of magnets 8 and 9 through the end pieces 11, 12, and 11, 12, into the rotary shaft and discs 7. A narrow gap of high magnetic flux density exists between the pole piece 10 and the edges of the discs 7. Accordingly, when a quantity of my improved developer is placed in the trough 14 and the shaft rotated, the upwardly, moving edges of the discs 7 adjacent the plate 16 attract and pick up the magnetic carrier particles and the powder adhering thereto, and move them into the high flux density magnetic gap and over the surface of the plate. Here, the powder is attracted to the oppositely charged latent electrostatic image and separated from the carrier particles which fall by gravity back into the trough 14 when they are carried beyond the region of high flux density in the magnetic gap. As the discs are inclined at an acute angle to the axis of rotation of the shaft, the edges of the disc brush the developer across the surface of the plate. At the same time, the plate is advanced vertically past the disc as shown by the arrow in Fig. 4 so that the developer will be applied over the entire surface of the charged plate.

As the powder is used up in operation additional powder must be added from time to time. Any means of fixing the resulting developed images known to those skilled in the art, such as those set forth in U. S. 2,297,691 to Chester F. Carlson, may be used.

A reversal reproduction of the original using the developer according to my invention may be easily obtained by applying the proper electric potential to the magnetic brush. Referring to Fig. 3, there is illustrated semidiagrammatically means for accomplishing this. These means include a battery 22 or similar D.-C. power source connected through a potentiometer 21 and electrical lead to shaft 4 which in turn is electrically connected to discs 7. Grounding sheet 15 places the electrostatic field across plate 16 when switch 23 is closed. I have obtained high quality reverse images by placing on the magnetic brush an electric potential of the same polar- A ity as in the electrostatic image and about twice the magnitude.

I have found that by the use of my invention improved solid area coverage is obtained in the image over that obtained if conventional methods were to be used. Even more unusual, either a positive or negative reproduction of the original may be obtained using the same developer merely by applying the proper electric potential to the magnetic brush. Finally, by using a single material as a developer, the advantages of handling and storing a single material are obtained.

While specific details of my invention have been described, many changes may be made in the details set forth without departing from the spiritand scope of my invention as defined in the appended claims.

I claim:

1. In a process wherein an electrostatic image is developed to yield an electrostatically adhering image of electrostatically attractable material, the improvement comprising contacting magnetic field producing means with an electroscopic developer powder consisting of a ferromagnetic ferrite of from about 100 to 325 mesh and having the composition MeFe O where Me is a bivalent metal whereby said powder is formed into brush-like streamers, the particles of said powder being electrostatically charged through triboelectric contact with each other, moving said field producing means relative to a surface bearing an electrostatic image so that said streamers contact said image, the electrostatic lines of force from said image attracting oppositely electrostatically charged particles from said streamers and depositing said particles on said surface to form on said surface a deposit of said powder faithfully corresponding to said image.

2. In a process wherein an electrostatic image is developed to yield an electrostatically adhering image of electrostatically attractable material, the improvement comprising contacting magnetic field producing means with an electroscopic developer powder consisting of a ferromagnetic ferrite of from about 100 to 325 mesh and having a specific resistance higher than 1,000 ohms-cm., an initial permeability not less than about and a maximum permeability not less than 183 whereby said powder is formed into brush-like streamers, the particles of said powder being electrostatically charged through triboelectric contact with each other, moving said field producing means relative to a surface bearing an electrostatic image so that said streamers contact said image, the electrostatic lines of force from said image attracting oppositely electrostatically charged particles from said streamers and depositing said particles on said surface to form on said surface a deposit of said powder faithfully corresponding to said image.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Young et al.: Electrofax, Reprint from R. C. A. Re

view, December 1954, vol. XV, No. 4, especially pages 471, 472, 480 and 481.

Berry et al.: F erromagnetography-High Speed, Gen eral Electric Review, July 1952, pages 20, 21, 22 and 61. 

1. IN A PROCESS WHEREIN AN ELECTROSTATIC IMAGE IS DEVELOPED TO YIELD AN ELECTROSTATICALLY ADHEREING IMAGE OF ELECTROSTATICALLY ATTRACTABLE MATERIAL, THE IMPROVEMENT COMPRISING CONTACTING MAGNETIC FIELD PRODUCING MEANS WITH AN ELECGROSCOPIC DEVELOPER POWDER CONSISTING OF A FERROMAGNETIC FERRITE OF FROM ABOUT 100 TO 325 MESH AND HAVING THE COMPOSITION MEFE2O4, WHERE ME IS A BIVALENT METAL WHEREBY SAID POWDER IS FORMED INTO BRUSH-LIKE STREAMERS, THE PARTICLES OF SAID POWDER BEING DELECTROSTATICALLY CHARGED THROUGH TRIBOELECTRIC CONTACT WITH EACH OTHER, MOVING SAID FIELD PRODUCING MEANS RELATIVE TO A SURFACE BEARING AN ELECTROSTATIC IMAGE SO THAT SAID STREAMERS CONTACT SAID IMAGE, THE ELECTROSTATIC LINES OF FORCE FROM SAID IMAGE ATTRACTING OPPOSITELY ELECTROSTATICALLY CHARGED PARTICLES FROM SAID STREAMERS AND DEPOSITING SAID PARTICLES ON SAID SURFACE TO FORM ON SAID SURFACE A DEPOSIT OF SAID POWDER FAITHFULLY CORRESPONDING TO SAID IMAGE. 